WINDOW SILLS AND DOOR THRESHOLDS Sealing the Cavity

The sill is the lower horizontal portion of a window opening and the threshold is the corresponding part of a door opening. The cavity at sill and threshold level is sealed by turning the internal blockwork across the cavity into the back of the external leaf of brickwork. A length of DPC felt (typically 200mm wide) is inserted at the back of the brickwork and turned over the top of the brickwork under the sill before positioning the frame. The length of felt should be sufficient so that it extends beyond the width of the opening by 150mm at both ends. This will ensure an
adequate 100mm overlap with any vertical DPC felt placed in both reveals of the opening. The length of felt used is, therefore, equal to the width of the opening plus 300mm.

Подпись: Damp Proofing at Openings It is vitally important around openings where the cavity is sealed with blockwork (and the cavity bridged) that an effective and continuous means of damp proofing is introduced into the construction, to maintain the integrity of the cavity and its purpose for being there. This applies as much to reveals and heads of openings as it does to sills and thresholds.

Unless aerated concrete blocks of the correct thermal grade are used, this form of construction is not regarded as good practice today because of the risk of ‘cold bridging’, which could cause internal condensation. To prevent cold bridging, insulation board or insulated DPC is introduced at the point where the blockwork seals across the cavity at sill level.

The most common and simplest sill detail is an integral sill where the sill forms the bottom part of the window frame. The groove at the front of the timber sill is called a ‘throating’ and acts as a drip to help prevent water running back under the sill and/or down the face of the brickwork. The drip encourages a build-up of water, the weight of which eventually overcomes its own surface tension, causing it to drop to the ground. The throating must, therefore, be positioned well clear of the wall. The throating of a sill works in much the same way as the drip on a cavity-wall tie. The sill of the frame is bedded in mortar on to the external leaf of brickwork in order to ensure level and even bearing on the brickwork.

WINDOW SILLS AND DOOR THRESHOLDS Sealing the Cavity

Fig. 178 Simple sill detail – uninsulated, resulting in ‘cold bridging’.

WINDOW SILLS AND DOOR THRESHOLDS Sealing the Cavity

Fig. 179 Simple sill detail – insulated, to prevent ‘cold bridging’.

Sub-Sills

An alternative to sills that are part of the frame inserted into an opening is to construct a structural sub-sill that provides a weathering at the bottom of the opening. A typical example is the use of a brick-on-edge sub-sill, which projects beyond the face of the main wall and is sloped so as to allow rainwater to fall clear of the wall below. Sometimes, specially shaped bricks such as ‘bull-nose’ or ‘cant’ bricks are used to make the sill detail more decorative. Adequate slope and projection is needed as it is not possible to create a throating under a brick-on-edge sub-sill.

WINDOW SILLS AND DOOR THRESHOLDS Sealing the Cavity

Fig. 180 Sill detail incorporating a brick-onedge sub-sill.

WINDOW SILLS AND DOOR THRESHOLDS Sealing the Cavity

Fig. 181 Sill detail incorporating a ‘cant’ brick sub-sill.

A DPC must be placed under the brick-on-edge, which is turned up at the point where the cavity is sealed to form a tray. The length of felt should be sufficient to extend beyond the width of the opening by 250mm at both ends. This will ensure an adequate 100mm overlap with any vertical DPC felt placed in both reveals of the opening, as wel as projecting by a further 150mm into the cavity at both ends. The length of felt used is, therefore, equal to the width of the opening plus 500mm. Alternatively, the tray can be provided with proprietary stop-ends or cloaks, which negate the need to extend the tray beyond the reveal and into the cavity.

In areas of severe or very severe exposure to rain, and especially when full-fill cavity insulation has been employed, it is a requirement to provide proprietary cloaks or stop-ends to cavity trays at sills instead of letting the tray terminate open-ended where it runs past the ends of the sill, into the vertical reveal and beyond into the cavity.

Insulation board is inserted where the blockwork closes off the cavity to prevent cold bridging. Alternatively, an insulated plastic cavity closer could be used (see Fig 184).

WINDOW SILLS AND DOOR THRESHOLDS Sealing the Cavity

Fig. 184 Insulated cavity closer installed at a window reveal.